The present invention relates to a fuel control system of an engine, particularly to a fuel control system of an engine which controls, by using a tumble flow, a behavior of fuel directly injected into a combustion chamber formed inside a cylinder of the engine.
When an operating state of an engine is within an operating range where an engine load is high and an engine speed is low, abnormal combustion, such as knocking and pre-ignition, easily occurs due to an inhomogeneous mixture gas and a slow flame propagation after spark ignition inside a combustion chamber of the engine. To suppress such abnormal combustion (e.g., knocking), it is known to significantly retard an ignition timing to after a top dead center of a compression stroke (CTDC). However, when the ignition timing is retarded to after the CTDC, combustion pressure inside the combustion chamber reduces, and thus, an output torque of the engine also reduces.
Therefore, an art of splitting a fuel injection into the combustion chamber into a plurality of injections to secure a high torque while suppressing knocking is proposed. For example, with a control method of an engine disclosed in JP4924751B, when an operating state of the engine is within a first engine speed range where the engine speed is relatively low, a retarding amount of an ignition timing is designed to be larger than that within a second engine speed range where the engine speed is higher than the first engine speed range, a fuel injection mode is designed to be a split injection mode in which the fuel is injected at least twice, and a timing of the final injection thereof is designed to be in an early half of the compression stroke. The fuel injected into the combustion chamber in the early half of the compression stroke reduces a temperature of mixture gas inside the combustion chamber by latent heat of vaporization. Thus, knocking can be suppressed and, accordingly, the ignition timing can be advanced to increase the torque.
Meanwhile, it is known that by increasing a flame propagation speed after spark ignition, a mixture gas can be combusted normally before knocking occurs, and thus, knock resistance improves.
However, with the control method of the engine in JP4924751B described above, the ignition is performed in a state where the fuel injected into the combustion chamber in the early half of the compression stroke is spread substantially uniformly inside the combustion chamber during the compression stroke. Therefore, compared to the case where the split injections of the fuel are not performed, there is no change in an air-fuel ratio of mixture gas near an ignition plug, and there is no significant difference in the flame propagation speed.
Therefore, in the conventional control method of the engine described above, there is a scope for improving the knock resistance even more by increasing the flame propagation speed, and advancing the ignition timing to increase the torque.